a) being born with a predisposition (which could mean an acquired immunity issue, a genetic tendency to one of the other known and unknown changes to the brain)
b) being born with some deformity that exhibits a loss or lessening of hypocretins
c) traumatic injury to the brain (physical blow, high fever, who knows?)
d) toxic or chemical event (environmental contaminants, surgical anaesthesia, drug reaction or overdose)
e) some completely unsubstantiated suspicions about survived extraordinary stressors.
since an immune or toxic reaction might be started via illness, one could guess at mosquito bites, even physical interaction with a parasite,
but that would be pretty wild conjecture, considering the very limited fund of data and understanding right now.
they still investigate such inane questions as "gee, why are you sleepy". it doesnt surprise me that there are idiotic assumptions about getting this from mosquitoes -- or from swinging a dead rat 3 times...